Subsidence analysis of the Barinas-Apure Basin: Western Venezuela

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1 Subsidence analysis of the Barinas-Apure Basin: Western Venezuela Maria Jácoe* Sión Bolívar University, Caracas, Venezuela and L. Chacín Halliburton, El Rosal, Caracas, Venezuela Suary A series of structural and isopach aps have been produced for the Barinas-Apure Basin, Western Venezuela, in order to deterine the regional distribution of the ain tectonostratigraphic units in the area, to quantify the original thickness of these units at the tie of their sedientation and to estiate the total baseent-driven subsidence of the basin. The structural and isopach aps were produced using fourteen 2D seisic regional transects and well log inforation provided by PDVSA. Six sequences were interpreted on the seisic lines since they reproduce best the tectonostratigraphic evolution of the basin, these are: Pre-Cretaceous (S6), Lower Cretaceous Capanian (S5), Maastrichtian-Paleocene (S4), Eocene-Lower Miocene (S3), Middle-Upper Miocene (S2) and Pliocene-Pleistocene (S1).The sedient thicknesses of these units then were decopacted using a 3D flexural decopaction software. Results show that S5 was deposited in a passive argin with a axiu subsidence rate (sr) of 21.4 /Ma, S4 arks the transition to a copressive regie associated with the collision of the Caribbean and South Aerican plates towards the northwest of the area (sr=38.2 /Ma), S3 was deposited in a foreland basin produced by the flexure of the western Venezuelan crust associated with the Lara Napes eplaceent (sr=47.5 /Ma) and S2 and S1 ark the foration of the Barinas-Apure foreland basin as the result of the eplaceent of the Mérida Andes, in were axiu subsidence rates increased considerably to /Ma for S2 and /Ma for S1. Introduction The Barinas-Apure Basin (BAB) is the third oil-producing basin of Venezuela. It has an extension of about 95 k² and a axiu sedient depth of about 5. This basin is located at western Venezuela to the southeast of the Mérida Andes (MA) range (Fig.1). The NE-SW trending ountain chain constitutes a large NE oriented uplifted block of about 42 k long that reaches a axiu topography of about 5 (Gonzalez de Juan, et al., 198). The Mérida Andes uplift is responsible for the BAB subsidence (Chacín, et al., 25). However the subsidence history of this basin related to the tectonic evolution of the area has not been quantitatively studied. Present day stratigraphic thicknesses are the result of copaction through tie, in order to quantitatively obtain the historical subsidence rates we need to decopact the units to their correct thicknesses at the tie of sedientation. Back to Exploration 28 CSPG CSEG CWLS Convention 682

2 15 Caribbean Plate Placa del Caribe Mbo AM Venezuela BA 5 Colobia Cratón de Guayana -5 5 15 1 Mérida Andes Barinas Apure Basin Figure 1: Location of the Barinas-Apure Basin: Western Venezuela Basin

(aprox. 62259 k 2, Fig. 1). Two way travel

These tops represent the ain tectonic events that affected the basin. Subsequently, decopaction was carried out (Flexural Decopaction Software 5.

2 2 15 Caribbean Plate Placa del Caribe Mbo AM Venezuela BA 5 Colobia Cratón de Guayana Mérida Andes Barinas Apure Basin Figure 1: Location of the Barinas-Apure Basin: Western Venezuela Basin Subsidence Six tectonostreatigraphic units were interpreted using 14 2D-regional seisic trasencts controlled with well log inforation, provided by PDVSA, covering the northern part of the basin (aprox k 2, Fig. 1). Two way travel tie and depth aps were then generated to the top of the intepreted units (i.e. Pre-Cretaceous (S6), Lower Cretaceous-Capanian (S5), Maastrichtian-Paleocene (S4), Eocene-Lower Miocene (S3), Middle-Upper Miocene (S2) and Pliocene-Pleistocene (S1)). These tops represent the ain tectonic events that affected the basin. Subsequently, decopaction was carried out (Flexural Decopaction Software 5.3 Badley Geoscience) to understand the burial history of the BAB and to deterine the original thickness of the tectonostratigraphic units. Decopacted thicknesses give inforation of the total subsidence associated with each stratigraphic unit, assuing that the accoodation space within the foreland basin is copletely filled with sedients (i.e. palaeo-water depth estiation near sea level). The ethod used to decopact the sedientary units assues a porosity depth function (Sclater and Christie, 198), in which porosity decays exponentially with depth. A regional 3D copilation of data covering thickness, age, porosity, lithospheric elastic thickness, density and lithology of each sedientary unit was required to be able to apply the decopaction technique. Results The decopacted isopach aps show (Fig. 2) that the ain subsidence in the basin is concentrated in two ain depocenters. One local depocenter is located towards the southwest of the basin and can be associated with a Jurassic rift structure (Lugo and Mann, 1995) another depocenter is located towards the northeast and can be related to regional tectonics. Back to Exploration 28 CSPG CSEG CWLS Convention 683

3 Fig. 2a shows the thickness of the Lower Cretaceous to Capanian unit (S5) is greater (i.e. 1 ) towards the southwest and this could be associated with a Pre-Cretaceous baseent structure. The north-eastern part of the basin shows a axiu thickness of thinning towards the south, with a axiu subsidence rate (sr) of about 24.1 /Ma (Table 1). This unit was deposited in a passive argin (Parnaud et al. 1995). Isopach ap of S4 sequence (Fig. 2b) shows two ain depocenters one located to the southwest () and the other one located towards the northeast (1 ) that could have been produced by the flexure of the South Aerican lithosphere as the result of the eplaceent of the Lara Nape (Parnaud, et al., 1995). The axiu subsidence rate (38.2 /Ma) is alost twice of that found in S5 iplying that S4 arks the transition between passive to active argin regie. The isopach ap of S3 shows a change in the direction of the subsidence fro NE-SW to alost E-W. During this tie the oriental cordillera of Colobia was eplaced (Parnaud, et al., 1995); this generated the accoodation space observed in this area (1 ) and increased the axiu subsidence rate (47.5 /Ma). During the Lower-Middle Miocene (S2) the eplaceent of the Mérida Andes is evident by the occurrence of a regional 22 thick depocenter adjacent and parallel to the deforation front (Fig. 2d). During this tie the subsidence rate increased considerably to /Ma. Plio-Pleistocene (S1, Fig. 2e) represents the axiu subsidence of the basin (337.7 /Ma) in which the tectonic load of the Mérida Andes was the greatest. The ain depocenter igrated fro the southwest to the northeast reaching a axiu thickness of 1. Back to Exploration 28 CSPG CSEG CWLS Convention 684

4 (a)thickness Lower Cretaceous - Capanean (S5) 2 (b) Thickness Maastrichtian - Paleocene (S4) (c) Thickness Eocene Lower Miocene (S3) 2 (d) Thickness Middle Upper Miocene (S2) 2 1 (e) Thickness Pliocene - Pleistocene (S1) Figure 2: Decopacted Isopach aps showing the true thicknesses of the tectonostratigraphic units interpreted fro the seisic lines. The units are: (a) Lower Cretaceous-Capanian (S5), Maastrichtian-Paleocene (S4), Eocene-Lower Miocene (S3), Middle-Upper Miocene (S2) and Pliocene-Pleistocene (S1). Maxiu subsidence rates are greatest during Middle-Upper Miocene (S2) and Pliocene-Pleistocene (S1), which corresponds to the Mérida Andes uplift. Maxiu Subsidence rates are shown in Table 1. Unit Maxiu decopacted thickness (k) Maxiu Subsidence rate (/Ma) Doinant tectonic subsidence echanis Lower Cretaceous -Capanian (S5) Passive argin Maastrichtian-Paleocene (S4) Transition to Lara Nappes thrust sheet loading Eocene Lower Miocene (S3) Lara Nappes thrust sheet loading Middle Upper Miocene (S2) Andean thrust sheet loading Pliocene Pleistocene (S1) Andean thrust sheet loading Table :1 Maxiu subsidence rates calculated for each tectonostratigraphic unit Back to Exploration 28 CSPG CSEG CWLS Convention 685

5 Conclusion A regional subsidence analysis, based on the true thickness, of the tectonostratigraphic units found in the Barinas-Apure Basin indicates that the Mérida Andes were uplifted in two ayor orogenic episodes: a first event took place towards the southwest during Middle to Upper Miocene which flexed the lithosphere and generated the accoodation space for the first Andean foreland-type sedients; afterwards a second and ore iportant uplift occurred towards the northeast of the basin, which generated enough subsidence to deposit 1 of Plio-Pleistocene sedients. Eocene to Lower Miocene unit was deposited over a caribbean-type foreland basin associated with the collision of the Caribbean and South Aerican plates towards the northwest of the basin. Maastrichtian-Paleocene unit represents the transition fro a passive argin regie to a Caribbean regional foreland basin subsidence. Lower Cretaceous unit was deposited over a passive argin. Acknowledgeents This work is part of the project PDVSA-USB-FUNINDES We are thankful to PDVSA (Petróleos de Venezuea, S.A.), for the inforation and resources provided for the developent of this study. References Chacín, L., Jácoe, M. I. and Izarra, C., 25, Flexural and gravity odelling of the Mérida Andes and Barinas-Apure Basin, Western Venezuela, Tectonophysics, 45, Gonzalez de Juana, C., Arozena, J. and Picard Cadillat, X., 198, Geología de Venezuela y de sus Cuencas Petrolíferas, Foninves (eds.), Caracas, 2, Lugo, J. and Mann, P., 1995, Jurassic-Eocene Tectonic Evolution of Maracaibo Basin, Venezuela, in A.J. Tankard, R. Suárez S. and H.J. Welsink, Petroleu basins of South Aerica: AAPG, 62, Parnaud, F., Gou, Y., Pascual, J.C, Capello, M.A., Truskowski, I. and Passalacqua, H., 1995, Stratigraphic synthesis of Western Venezuela, in A.J. Tankard, R. Suárez S. and H.J. Welsink, Petroleu basins of South Aerica: AAPG, 62, Sclater, J.G. and Christie, P.A. F., 198, Continental stretching: an explanation of the post-id Cretaceous subsidence of the North Sea Basin. Journal of Geophysical Research, 85, Back to Exploration 28 CSPG CSEG CWLS Convention 686

Figure 1. Examples of vector displacement diagrams for two and three-plate systems.

Figure 1. Examples of vector displacement diagrams for two and three-plate systems. Figure 2. Relationships between pole of rotation, great circles, ridge segments, small circles, transforms and fracture